Terminals perform PLMN selection for each access technology as if it were the only access technology the terminal is capable of connecting to; this selection is not influenced by the fact that the terminal has a multi-interface capability. There are existing standards specifications defining PLMN selection procedures for 3GPP accesses, and TS 22.011 (v. 8.9.0), 3GPP, Technical Specification Group Services and Systems Aspects, Service Accessibility (Release 9) specifies: “As consequence, the multi-mode terminal when entering 3GPP mode of operation shall act as if it were a 3GPP only UE [user equipment] which had just been switched-on. Similarly, when leaving the 3GPP mode of operation the multimode terminal shall act as if it were a 3GPP only UE which had just been switched-off”.
Simultaneous connectivity over a 3GPP and a non-3GPP access is supported in current specifications (see TS 23.402 (v.9.3.0) 3GPP Technical Specification Group Services and System Aspects; Architecture enhancements for non-3GPP accesses). PLMN selection for 3GPP and non-3GPP accesses is performed in an independent manner (e.g., PLMN selection for 3GPP access [TS 22.011 (v.9.3.0) 3GPP Technical Specification Group Services and System Aspects; Architecture enhancements for non-3GPP accesses] and PLMN selection of I-WLAN [TS 23.234 (v.9.0.0) 3GPP Technical Specification Group Services and System Aspects; 3GPP system to Wireless Local Area Network (WLAN) interworking; System description and TS 24.234 (v. 8.3.0), 3GPP, Technical Specification Group Core Network and Terminals, 3GPP System to Wireless Local Area Network (WLAN) Interworking; WLAN User Equipment (WLAN UE) to network protocols (Stage 3)]. There is no shared information between PLMN selection procedures for 3GPP and non-3GPP accesses. Instead, the list of preferred PLMNs for 3GPP access is stored separately from the list of preferred PLMNs for non-3GPP access, e.g., in USIM, PLMN record information is stored separately from I-WLAN PLMN records (see TS 23.008 (v.9.1.0) 3GPP Technical Specification Group Core Network and Terminals; Organization of subscriber data). The last registered PLMN for 3GPP access is stored in the SIM/USIM as a different record from the last registered PLMN for I-WLAN (see TS 23.008).
3GPP also standardizes PLMN selection procedures for non-3GPP access for I-WLAN (see TS 22.234, (v. 8.1.0), 3GPP, Technical Specification Group Core Network and Terminals, Requirements on 3GPP System to Wireless Local Area Network Interworking (Release 7) and TS 24.234). Separate from 3GPP accesses, there is no statement saying that PLMN selection procedures for I-WLAN do not depend on the terminal having multi-mode capabilities. The procedures defined, however, do not link PLMN selection procedures for I-WLAN with PLMN selection procedures for 3GPP accesses.
Therefore, with current standards, PLMN selection procedures for I-WLAN and PLMN selection procedures for 3GPP accesses are performed in an independent manner. It is then possible for a multi-mode UE to connect to different PLMNs over different accesses, for example, one for 3GPP access and a different one for non-3GPP accesses. In such scenarios, flow mobility among access networks may be limited due to the fact that network policies can only be applied within an operator's domain.
PLMN selection procedures have been defined independently for each access technology type, so that the PLMN selected on an access technology type cannot influence the PLMN selected on a different access technology type. Simultaneous connectivity over different 3GPP accesses is not supported in current specifications. 3GPP PLMN selection procedures are only defined when the UE is connected to only one 3GPP access at a time. A preferred PLMN list can be access type specific, meaning that the list of preferred PLMNs for access type i may be different from the list of preferred PLMNs for access type j.
However, when the network sends policies to the UE to steer traffic from one access network to another, e.g., Internet offload to WLAN or IP flow mobility, such policies can only take effect among access networks that belong to the same PLMN (or equivalent PLMN). This makes network controlled mobility (or steering of access) between access technologies difficult. In other words, it is not possible for operators to offload traffic from their networks from one access technology type to another, if these types are not connected on the UE to the same operator.
Accordingly, a multi-interface terminal selects PLMN for each access type (each interface may be associated with a different access type) in an independent manner, i.e., the fact that a PLMN has been selected for a particular access type is not taken into account in the PLMN selection of other access types. If the terminal is capable of transmitting simultaneously over multiple interfaces, this independent selection process may result in the terminal being simultaneously connected to multiple PLMNs. In such a case, an operator cannot offload traffic from one access type to another, as operator's policies can only be applied within its own domain (i.e. within the same PLMN). For example, if a terminal is connected to OP1 over 3GPP access and OP2 over WLAN, OP1 cannot offload traffic to WLAN, as this would mean the traffic would be offloaded to a different operator. Traffic offload to alternate access is becoming very important for cellular operators as bandwidth hungry data applications are growing.
Thus a generalized PLMN selection mechanism that considers multiple-interface capability and network policy is needed to better manage the flow mobility. Having generalized procedures of PLMN selection for multi-interface terminals will allow operators to better manage the flows across access networks through network policies.